Fuel cells using solid polymer electrolyte membranes are expected to find a wide variety of commercial applications as a power source or simple auxiliary power source for electric vehicles because of a low operating temperature below 100° C. and a high energy density. Involved in these PEFCs are important elemental technologies relating to such elements as electrolyte membrane, platinum based catalyst, gas diffusion electrode, and electrolyte membrane-electrode assembly. Among these, the technology relating to electrolyte membrane and electrolyte membrane-electrode assembly is one of the most important technologies governing the fuel cell performance.
In PEFCs, a fuel diffusion electrode and an air diffusion electrode are joined to opposing surfaces of an electrolyte membrane, so that the electrolyte membrane and electrodes form a substantially integral structure. Then the electrolyte membrane functions as an electrolyte for conducting protons and also plays the role of a diaphragm for preventing direct intermixing between hydrogen or methanol as the fuel and air or oxygen as the oxidant even under pressure.
Such electrolyte membranes are required as the electrolyte to have a high rate of proton transfer, a high ion exchange capacity, and a consistent, high water retention to maintain a low electric resistivity. From the role of a diaphragm on the other hand, electrolyte membranes are required as the membrane to have a high mechanical strength, dimensional stability and chemical stability in long-term service, and to rid of excessive permeability to hydrogen gas or methanol as the fuel and oxygen gas as the oxidant.
At the present, perfluorosulfonic acid/fluorocarbon resin membranes developed by E.I. duPont and commercially available as Nafion® are generally used. Conventional fluorocarbon resin electrolyte membranes as typified by Nafion® suffer from the problem of increased cost due to a number of steps involved in the manufacture process which has to start from the synthesis of monomers, and this problem becomes a serious bar to commercial application.
Efforts have thus been made to develop low-cost electrolyte membranes as a substitute for Nafion® and analogues. With respect to radiation-induced graft polymerization, JP-A 2002-313364 and JP-A 2003-82129 propose a method for producing a solid polymer electrolyte membrane by irradiating a fluorocarbon resin membrane with radiation to create radically active sites in the fluorocarbon resin, and grafting a reactive hydrocarbon monomer thereto, followed by sulfonation.
The membrane obtained from graft polymerization of a reactive hydrocarbon monomer by a radiation-induced graft polymerization process has a high degree of grafting and hence, a high proton conductivity, but suffers from lack of oxidation resistance.
While the radiation used in the conventional radiation-induced graft polymerization process is electron beam or gamma-ray, no reports about ultraviolet-induced graft polymerization have been found. It is believed that UV irradiation fails to induce graft polymerization to fluorocarbon resins having C—F bonds.